Abstract

Subsurface lithologic and geophysical data from boreholes at the Pantex Plant in Carson County, Texas, reveal the extent of Triassic Dockum Group strata preserved in a local syndepositional basin, the center of which lies northeast of the Pantex site. It is here referred to as the Carson County Basin. Cuttings and some core from these boreholes contain lithologies correlative to the Trujillo Sandstone, the Tecovas Formation, and the Camp Springs Member of the Tecovas Formation that crop out along the Canadian River and its tributaries near Pantex. The Camp Springs Member of the Tecovas Formation is a thin fluvial sandstone and conglomerate deposited in paleochannels eroded into the upper part of the Permian Quartermaster Formation. The overlying strata of the Tecovas Formation consists of shale, siltstone, and interbedded sandstone deposited on a low-relief flood plain. The Trujillo Sandstone consists of fluvial sandstone, siltstone, shale and minor conglomerate deposited by a higher energy river system. Dockum Group isochore maps show Triassic strata thickening towards the basin center, indicating syndepositional basin development. Cross sections show the Trujillo Sandstone is thicker than the Tecovas Formation towards the basin center, indicating an increased subsidence rate for the Trujillo Sandstone that potentially resulted in higher energy sedimentation. The coarser clastic rocks of the Trujillo Sandstone form a more productive horizon of the Dockum Aquifer than the relatively tight shale in the subjacent Tecovas Formation. A structure contour map of the top of the Dockum Group defines the pre-Ogallala erosional surface as a dendritic river system that drained to the east. Irregular basal Dockum Group surfaces associated with the modern playas supports other studies that indicate evaporite dissolution within deeper Permian strata resulted in basin subsidence in the Texas Panhandle during the Late Triassic.